Re: acoustic imager

On 2025-04-16 10:41, john larkin wrote:

On Wed, 16 Apr 2025 09:01:00 +0100, Cursitor Doom <cd@notformail.com>
wrote:
 

On Tue, 15 Apr 2025 15:04:15 -0700, john larkin <jl@glen--canyon.com>
wrote:
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https://www.google.com/aclk?sa=L&ai=DChsSEwjTjaDVg9uMAxW3Hq0GHVmKOlYYACICCAEQARoCcHY&co=1&cce=2&sig=AOD64_3aGs74magNuXwdRGFo7oP8zK-LMQ&ctype=5&q=&adurl=

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For 42,000 dollars? There's a product there you could develop, John.

 Seems like it needs maybe a dozen electret mikes, one mux'd ADC, an
FPGA, and some code.

In the last few decades, there's been a lot of work done on imaging with sparse arrays.
A full NxN rectangular antenna array has an enormous amount of duplicated information from an imaging point of view. To make a good image, you need spatial frequency information corresponding to all values of dx and dy, with some regular spacing, i.e. in an NxN array,
dx and dy go from -N/2 to +N/2-1 (or equivalently, from 0 to N-1) in integer steps.
In principle you only need one estimate per spacing, but in a dense array, every pair of adjacent pixels gives an estimate of the dx = +-1 components, i.e. essentially the same information as every other adjacent pair.  The redundancy is less at wider spacing, of course.
If one is willing to trade off SNR and computational expense, you can get the resolution of a full array with far less than N**2 antennas--I forget what the the number is, but it's a lot more like N log N than N**2.  A pal of mine in grad school, Yoram Bresler, did his thesis on that problem, which is where I first heard of it.
So a sparse array of microphones can in principle do quite a bit better than one might suppose.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com